Stacks and Queues

Queue Enqueue/Dequeue

Enqueue values at the back and dequeue them from the front in first-in, first-out order.

Algorithm

The replay uses the same three values in every language, so this Ruby DSA implementation can be compared directly with the rest of the DSA track.

front The front is the oldest value still waiting in the queue.
FIFO A queue removes values in first-in, first-out order.

Visual walkthrough

The queue keeps the oldest value at the front and adds new values at the back.

Step 1 - Enqueue 10, 20, 30

New values join at the back. The oldest value, 10, stays at the front.

Queue after three enqueues: front 10, then 20, then back 30.nextnext10front2030back

Step 2 - Dequeue removes 10

Removing from the front returns 10 and makes 20 the new front.

After one dequeue: removed is 10; front moves to 20.next10removed20front30back

Basic Implementation

basic.rb 
queue = []
[10, 20, 30].each { |value| queue.push(value) }
removed = []
removed << queue.shift until queue.empty?
puts removed.join(" -> ")

Complexity

  • Time: O(1) per operation with a real queue
  • Space: O(n)

Implementation notes

  • The queue is a Ruby Array, starting as queue = [].
  • Enqueue uses queue.push(value) inside [10, 20, 30].each, so new values are appended at the back in that order.
  • Dequeue uses queue.shift, which removes and returns the front element while mutating the array.
  • removed << queue.shift until queue.empty? keeps shifting only while the queue has values, so this source never relies on shift returning nil from an empty array.
  • FIFO order is visible in the trace: [10, 20, 30] becomes [20, 30] after removing 10, then becomes empty after removing 20 and 30.
  • removed is another Ruby array that records the returned values as [10, 20, 30].
  • puts removed.join(" -> ") prints the deterministic output 10 -> 20 -> 30.